1. Field of the Invention
The present invention relates, generally to the control of a dual clutch transmission and, more specifically, to a method for controlling the speed of the engine by controlling the torque transfer of the clutches of a dual clutch transmission as used in a motor vehicle.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the xe2x80x9ctransmission.xe2x80x9d Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Presently, there are two typical transmissions widely available for use in conventional motor vehicles. The first, and oldest type is the manually operated transmission. These transmissions include a foot operated start-up or launch clutch to engage and disengage the driveline with the power plant and a gearshift lever to selectively change the gear ratios within the transmission. When driving a vehicle having a manual transmission, the driver must coordinate the operation of the clutch pedal, the gearshift lever and the accelerator pedal to achieve a smooth and efficient shift from one gear to the next. The structure of a manual transmission is simple and robust and provides good fuel economy by having a direct power connection from the engine to the final drive wheels of the vehicle. Additionally, since the operator is given complete control over the timing of the shifts, the operator is able to dynamically adjust the shifting process so that the vehicle can be driven most efficiently. The disadvantages of the manual transmission is that there is an interruption in the drive connection during gear shifting and that there is a great deal of required physical interaction on the part of the operator to shift gears.
The second, and newer choice for the transmission of power in a conventional motor vehicle is an automatic transmission. First and foremost, automatic transmissions offer ease of operation. The driver of a vehicle having an automatic transmission is not required to use both hands, one for the steering wheel and one for the gearshift, and both feet, one for the clutch and one for the accelerator and brake pedal in order to safely operate the vehicle. In addition, an automatic transmission provides greater convenience in stop and go situations, because the driver is not concerned about continuously shifting gears to adjust to the ever-changing speed of traffic. Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, they suffer from the disadvantage of reduced efficiency because of the need for hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween.
At low speed ratios, RPM output/RPM input, torque converters multiply or increase the torque translation from the engine. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, at high speed ratios there is no torque multiplication and the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (RPM input greater than than RPM output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the entire powertrain. Further, the torque converter itself requires pressurized hydraulic fluid in addition to any pressurized fluid requirements for the actuation of the gear shifting operations. This means that an automatic transmission must have a large capacity pump to provide the necessary hydraulic pressure for both converter engagement and shift changes. The power required to drive the pump and pressurize the fluid introduces additional parasitic losses of efficiency in the automatic transmission.
In an ongoing attempt to provide a vehicle transmission that has the advantages of both types of transmissions with fewer of the drawbacks, combinations of the traditional xe2x80x9cmanualxe2x80x9d and xe2x80x9cautomaticxe2x80x9d transmissions have evolved. Most recently, xe2x80x9cautomatedxe2x80x9d variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated manual transmissions typically include a plurality of power-operated actuators that are controlled by a transmission controller or some type of electronic control unit (ECU) to automatically shift synchronized clutches that control the engagement of meshed gear wheels traditionally found in manual transmissions. The design variants have included either electrically or hydraulically powered actuators to affect the gear changes. However, even with the inherent improvements of these newer automated transmissions, they still have the disadvantage of a power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a harsh shift feel which is generally considered to be unacceptable when compared to smooth shift feel associated with most conventional automatic transmissions.
To overcome this problem, other automated manual type transmissions have been developed which can be power-shifted to permit gearshifts to be made under load. Examples of such power-shifted automated manual transmissions are shown in U.S. Pat. No. 5,711,409 issued on Jan. 27, 1998 to Murata for a Twin-Clutch Type Transmission, and U.S. Pat. No. 5,966,989 issued on Apr. 04, 2000 to Reed, Jr. et al for an Electro-mechanical Automatic Transmission having Dual Input Shafts. These particular variant types of automated manual transmissions have two clutches and are generally referred to simply as dual, or twin, clutch transmissions. The dual clutch stricture is most often coaxially and cooperatively configured so as to derive power input from a singular engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electro-mechanical type, since a lubrication system within the transmission is still a necessity requiring a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. These pumps are most often gerotor types, and are much smaller than those used in conventional automatic transmissions because they typically do not have to supply a torque converter. Thus, any parasitic losses are kept small. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform up-shifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear engagement in a dual clutch transmission is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. To automate the transmission, the mechanical selection of each of the gear sets is typically performed by some type of actuator that moves the synchronizers. A reverse gear set includes a gear on one of the input shafts, a gear on the counter shaft, and an intermediate gear mounted on a separate counter shaft meshingly disposed between the two so that reverse movement of the output shaft may be achieved.
While these power-shift dual clutch transmissions overcome several drawbacks associated with conventional transmissions and the newer automated manual transmissions, it has been found that controlling and regulating the automatically actuated dual clutch transmissions is a complicated matter and that the desired vehicle occupant comfort goals have not been achievable in the past. There are a large number of events to properly time and execute within the transmission to achieve smooth and efficient operation. Not only during the power-shifting events but also throughout the entire operating range of the transmission as well. To this point, conventional control schemes and methods have generally failed to provide this capability. Accordingly, there exists a need in the related art for better methods of controlling the operation of dual clutch transmissions.
One particular area of control improvement that is needed is in the control of the vehicle engine speed through the control of the torque transferred across of the clutches of the transmission. The nature of the dual clutch transmission, that is, the manual style configuration discussed above, which employs disc type clutches that are automatically actuated, requires accurate control of the clutch engagement and thus the torque transferred across them. More specifically, it is desirable to operate the clutches of the dual clutch transmission so that engine speed is controlled by varying the amount of torque transferred across the clutch, or in other words to induce certain amounts of clutch slip in certain parts of the vehicle""s operating range.
The control schemes for dual clutch transmissions known in the related art are incapable of adequately providing for fine control of engine acceleration to satisfy this need. Specifically, they lack the ability to finely control the torque transferred across the clutches to achieve the high degree of accuracy needed for smooth transmission and engine operation. Additionally, current control methods for the clutches of a dual clutch transmission generally concern themselves with simple engagement and disengagement of the clutch assemblies and fail to adequately provide for the corresponding control of the speed of the vehicle engine. This causes adverse engine responses to clutch engagement, such as engine lugging or over-revving. The lugging effect occurring when a clutch is heavily engaged without adequate engine speed, and the engine becomes excessively loaded, causing surges and roughness. The over-revving effect occurring when the clutch engagement is slow and behind the acceleration of the engine so that the slip is excessive and power is lost.
Other current control methods employ a rudimentary engine speed control of the type most associated with single clutch automated manual transmissions and which are not well suited to dual clutch transmissions. For example, these conventional speed control methods utilize only a static engine target speed for engine control. This causes the vehicle operator to either continually increase the throttle position to command a higher engine speed which results in the engine speed being controlled in distinct and noticeable stages, or to put the throttle to its highest setting each time an increase in engine speed is desired to achieve continued engine acceleration. While this type of engine speed control maybe acceptable in certain ranges of the engines operation, it is inadequate to employ this control over the engine speed throughout its entire range when using a dual clutch transmission. Accordingly, there remains a need in the art for a method to control the clutches of a dual clutch transmission to control the torque transferred so that, as the engine is commanded to accelerate, the change in engine speed matches a predetermined engine speed target curve.
Additionally, the current speed control methods employed with dual clutch transmissions fail to account for changes in driving conditions that effect the torque transfer across the clutches that is being used for speed control. This results in false responses to the target engine speed so that the desired engine speed cannot be achieved. Accordingly, there remains a need in the art for a method to operatively and actively control the speed of the engine by controlling the torque transfer of the clutches of a dual clutch to cause the engine to track the target engine speed in response to changes in the driving conditions.
The disadvantages of the related art are overcome by a method of controlling the pressure applied to the engaged clutch of a vehicle having a dual clutch transmission to control the torque transferred across the engaged clutch thereby providing engine speed control for each gear based on the engine throttle position. The method includes the steps of determining the engine throttle position, determining the currently engaged gear of the transmission, and sensing the speed of the clutch. An engine stall speed is selected for the current gear and engine throttle position from a look-up table, and then a target engine speed based on the engine stall speed and the clutch speed is continuously redetermined, the clutch speed increasing in response to the increasing engine speed. The method also includes determining the difference between the target engine speed and the actual engine speed and then varying the torque transferred across the clutch driving the engaged gear to cause the engine to track the target engine speed based on the difference between the target engine speed and the actual engine speed. Thus, the engaged clutch is operatively slipped to smoothly cause the engine to track a predetermined target speed, providing efficient operation with the desired smooth driving feel. In this way, hard clutch lock-ups and engine lugging are avoided.
Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description taken in connection with the accompanying drawings.